The Main Fossiliferous Layer (MFL) is a concentration of vertebrate and invertebrate fossils, 20 to 30 cm thick, preserved in a sequence of glauconitic sand at or near the Cretaceous–Paleogene boundary in the New Jersey (USA) coastal plain. Several hypotheses have been proposed to explain the origin and age of the MFL, including: marine transgression and formation of lag deposits of reworked bones and shells, formation of a condensed section and attritional accumulation of fossil material, and catastrophic collapse of Late Cretaceous ecosystems following the end-Cretaceous bolide impact at Chicxulub. We use new data on glauconite morphology, concentrations, geochemistry, and presence of shocked quartz, coupled with previous data on sedimentology, taphonomy, and rare-earth-element geochemistry of fossil vertebrates to interpret the genesis of the MFL. Glauconite concentration and maturity steadily increases from latest Cretaceous sediments of the Navesink–New Egypt Formation, through the MFL and into the Paleogene upper Hornerstown Formation, suggesting that marine transgression and decreasing sedimentation rates were a factor in the formation of the glauconite, but not the MFL. Rare-earth-element patterns in fossil bones from the MFL, which are acquired during fossilization, are different from those of Cretaceous and Paleogene specimens, indicating that vertebrate remains in the MFL fossilized in situ and were not reworked from older, underlying units. Vertebrate fossils from the MFL are preserved as isolated to articulated specimens. While isolated specimens would be common in a transgressive lag, articulated specimens would not. Articulated specimens could be concentrated by attritional accumulation along a defined surface during a period of slow sedimentation, but the lack of a distinct increase in glauconite maturity or concentrations of elements associated with heavy-mineral accumulations at the Navesink–New Egypt–Hornerstown contact or MFL, which would be expected during a period of reduced sedimentation, hiatus, or unconformity, are absent at the localities studied. Shocked quartz was identified in a burrow fill directly beneath the MFL, at the contact of the Navesink and Hornerstown formations. Clay clasts with latest Cretaceous microfossils, along with reworked invertebrate fossils with infills of latest Cretaceous sediment, have been recovered from the MFL. The association of shocked quartz, mixture of isolated and articulated vertebrates with distinct rare-earth signatures, lack of a punctuated period of increased glauconite maturity, and presence of reworked Late Cretaceous invertebrates and clay clasts with Late Cretaceous microfossils suggests that the MFL may represent a thanatocoenosis that was the direct result of environmental disturbance associated with the end-Cretaceous impact event at Chicxulub.

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